Ultrasonic Beam Expander

Abstract

A single compact unit for generating and controlling an ultrasonic beam which includes an ultrasonic crystal generator and a solid ultrasonic lens with a liquid ultrasonic energy coupling therebetween which is contained in a housing joining the crystal and the lens. An antireflection coating on the ultrasonic lens surfaces improves transmission efficiency.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to ultrasonic generators and more specifically to those types of generators which include means for controlling the wavefront curvature of the output beam.

Beamed ultrasonic energy is used for many purposes, such as for examination of various types of objects for hidden flaws therein. The usual source of ultrasonic energy is a piezoelectric transducer driven by an electronic oscillator at an appropriate frequency. Such transducers can be manufactured from a wide variety of materials. Quartz transducers are often preferred for high quality wavefronts. Unaided by other elements, an ultrasonic energy beam of large cross-sectional area is obtained by a transducer of substantially the same large area. Transducers, especially those made of quartz, are very expensive and cost an amount which varies in proportion to their area. Therefore, it is desirable to use as small a transducer as possible and somehow expand the small ultrasonic energy beam emitted therefrom into a beam having a larger cross-sectional area without degrading the wavefront quality.

Such expansion has been accomplished in the past by a combination of two or more ultrasonic lens elements independent of the transducer, or by a curved transducer and an ultrasonic lens. A general discussion of ultrasonic lenses may be had by reference to the book Sonics, authored by Heuter & Bolt, 1955, page 265. Use of a lens element separate from a transducer is inconvenient to mechanically support in a manner to prevent misalignment which may affect beam spread and also affect energy density of the beam.

Therefore, it is a primary object of this invention to provide a compact unitary transducer-beam control package for generating an ultrasonic beam having a larger cross-sectional area than the ultrasonic beam emitted by the transducer.

It is also an object of this invention to provide an ultrasonic beam lens element with a high refractive power but with reduced losses from energy reflection.

SUMMARY OF THE INVENTION

These and additional objects are accomplished by an ultrasonic energy generator according to the present invention which includes a transducer and a single element solid ultrasonic lens, both supported by a housing which contains a liquid for coupling ultrasonic energy therebetween. The transducer is preferably flat X-cut quartz which emits a collimated ultrasonic beam which strikes the lens at an incident surface thereof. The radius of curvature of the incident lens surface is chosen to substantially increase the divergence of the incident beam. The divergence of the beam passing through the lens element and the thickness of the lens determine the cross-sectional dimension of the ultrasonic beam emitted through an exit surface of the lens. The exit surface has a radius of curvature chosen to converge to some degree the diverging beam passing through the ultrasonic lens and thereby produce an output ultrasonic beam having a predetermined wavefront curvature and cross-sectional area. The output beam is, therefore, substantially larger in cross-sectional area than the incident ultrasonic beam generated by the quartz transducer alone.

Such an ultrasonic energy generator will most commonly be used in a liquid such as ordinary water or a liquid having a velocity of sound therein that is close to that of water. Therefore, the lens element should have the characteristic that sound travels therein much faster than it does in water so that the radii of curvature of the lens surfaces may be kept large to avoid generation of unwanted shear waves. The coupling liquid within the housing may have a speed of sound lower than that of water so that the radius of curvature of the incident lens surface may be kept large. With materials characterized by these relative speeds of sound therein, the incident lens surface is convex and the exit lens surface is concave. A preferred combination of materials satisfying these criteria is aluminum or an alloy with high aluminum content for the lens element and a halogenated hydrocarbon such as trichlorotrifluoro-ethane (commercially available as Freon 113) or carbon tetrachloride for the coupling liquid within the generator's housing.

According to another aspect of the present invention, it has been found that the normally high reflection loses at the boundaries of an aluminum lens may be substantially reduced by applying thereto a coating of graphite having a thickness at any point along the surfaces of the lens equal to an odd number of quarter wavelengths of ultrasonic energy in the graphite. So that an ultrasonic energy generator according to this invention will have low reflection losses from the lens element and will be operable at the fundamental transducer frequency at any of its odd harmonics, a thickness of graphite that is equal to a quarter wavelength at the fundamental frequency is applied to both sides thereof.

For a more detailed disclosure of a specific form of the present invention and for further objects and advantages thereof, reference should be had to the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in one form a beam expanding ultrasonic generator according to the present invention;

FIG. 2 is a cross-sectional view of the ultrasonic generator of FIG. 1 cut in half along a plane passing through section 2--2;

FIG. 3 illustrates a modification of the ultrasonic generator shown in cross section in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Use of the term "ultrasonic energy" herein is not to be taken as a limitation upon the frequency range in which a beam expanding ultrasonic generator according to the present invention may operate. The techniques of the present invention apply to generating and controlling compressional wave energy of any useable frequency. However, in the more practical applications of compressional wave energy, such as an object inspection, the higher sonic frequencies (i.e. those considerably above the audible range) are more desirable than the lower sonic frequencies. For this reason, instead of utilizing the more general term "compressional wave energy," the term "ultrasonic energy" will be utilized in describing the present invention. This should, however, in no way limit the scope of the invention.

The specific example of the invention described herein with respect to the figures is an ultrasonic energy generator utilizing a piezoelectric transducer which generates a collimated ultrasonic energy beam of one cross-sectional diameter, which beam is incident upon an ultrasonic lens which emits a collimated beam of ultrasonic energy having a much greater diameter. Since a device with these characteristics has great advantages in ultrasonic work, it is described in some detail herein but, as may be noted in the description hereinafter, the scope of applicant's invention is not limited to the specific embodiment described herein. Referring to FIG. 1, a beam expanding lens 11 is attached to a transducer containing housing 13 and generates a collimated ultrasonic beam through an exit surface 15 of the lens 11.

FIG. 2 shows an enlarged cross-sectional view of the ultrasonic generator and beam expander of FIG. 1. A flat X-cut quartz piezoelectric transducer 17 is physically attached to the housing 13 in some appropriate liquid-tight manner. The transducer 17 is electrically connected to an electronic oscillator (not shown) for generating a collimated beam 19 having a cross-sectional area substantially corresponding to the area of the transducer 17. The collimated beam 19 having a radius h strikes the lens element 11 at an incident surface 21 thereof. The incident surface 21 is convex with a radius of curvature r.sub.1 appropriate for the particular materials used to diverge the rays of the collimated beam 19 into a diverging ultrasonic energy beam 23. The lens 11 has a thickness d along its center line. The exit lens surface 15 has a radius of curvature r.sub.2 appropriate for the particular materials used to converge the diverging beam 23 into a collimated output beam 25 having a radius h". A useful beam expander for a wide variety of testing applications is one utilizing a readily available and economical 2 inch quartz crystal 17 (h = 1 inch) which emits a 6 inch output beam 25 (n" = 3 inches).

The housing 13 contains an ultrasonic coupling liquid 27 to efficiently transfer energy from the transducer to the lens. Use of a liquid coupling medium instead of a solid medium provides flexibility in chosing the shapes of the transducer 17 and of the incident lens surface 21.

In order to keep the beam expanding ultrasonic generator as compact as possible, care is chosen in selecting the materials of the coupling liquid 27 and of the solid lens 11. It is assumed for purposes of describing herein a specific embodiment of the invention that the beam expanding ultrasonic generator will be used in a water medium (n" = 1.0). Aluminum is a preferred material for the solid lens 11 since it has a low index of refraction relative to water, n' = 0.235 (defined as the ratio of velocity of sound in water to the velocity of sound in aluminum). The coupling liquid 27 is chosen from among those which have index of fraction much greater than that of water, and thus one that is useful in combination with an alumunum lens. Such a liquid is trichloro-trifluoroethane (commercially available as Freon 113), with an index of refraction of n = 2.066. The large difference between the indexes of refraction of aluminum and Freon, and between the indexes of refraction of aluminum and water allows the radius of curvature of the lens surfaces to be kept large as well as allowing the thickness of the lens to remain small for a given beam expansion.

Certain well known optical principles may be applied in the design of a specific ultrasonic beam expander. Applicable optical principles are discussed in a number of texts, such as Fundamentals of Optics by Jenkins and White, third edition, especially Chapter 8 thereof. Referring to FIG. 2, a method of design may be illustrated by tracing an extreme ultrasonic ray 20 from the transducer 17, through the lens 11 and into the output ultrasonic beam 25.

The ray 20 strikes the incident lens surface 21 at an angle .theta. with a line normal to the surface. The ray 20 emerges from that surface at an angle .theta.' with the normal. These angles are related by Snell's law of refraction, as follows:

sin .theta.' = (n/n') sin .theta. (1)

For a compact beam expanding unit, the angle .theta.' should be made as large as possible. However, as (sin .theta.') approaches unity the intensity of undesired shear waves becomes very large and the transmitted wave decreases in proportion until at unity all energy reflection is total, which results in poor efficiency of the lens. Therefore, (sin .theta.') is chosen to be,

sin .theta.' = K.sub.1 (2)

where K.sub.1 is some safety factor less than unity. For instance, K.sub.1 could be chosen to be 0.75 but its particular value will depend upon the lens material used and the amount of losses that can be tolerated.

From the geometry of FIG. 2, it is seen that,

sin .theta. = h/r.sub.1

Combining equations(1), (2) and (3) gives an expression for determining the radius of curvature r.sub.1 of the incident lens surface 21:

r.sub.1 = n h/n' K.sub.1 (4)

The next step in designing a beam expander according to the present invention is to determine the thickness of the lens 11 required to produce a collimated output beam 25 of the desired diameter. A graphical method is satisfactory. The angle .theta.' can be calculated from equation (2). This establishes the path of the extreme ray 20 through the lens, and also establishes the path of an extreme ray 22. Ray 22 follows a path which is the mirror image of the path of ray 20. These two extreme rays are graphically extended until they are separated by an amount 2h", the desired output beam diameter. This separation is shown in FIG. 2 between points 23a and 23b on the extreme rays. Two points of the lens exit surface 15 are then established to be points 23a and 23b.

The next step is to determine a radius of curvature r.sub.2 of the lens exit surface 15 which will cause the extreme ray 20 to leave the lens substantially parallel with the center line, thereby to produce the collimated ultrasonic beam 25. The ray 20 strikes the exit surface 15 at an angle .theta." with a line normal to the surface 15. The ray 20 leaves the exit surface 15 at an angle .theta..sub.2 with the normal. These angles are related by Snell's law of refraction, as follows:

sin .theta..sub.2 = (n'/n") sin

From the geometry of FIG. 2, the maximum value of the angle .theta." is determined by,

.theta." = .theta..sub.2 + .theta.' - .theta. (6)

where .theta.' and .theta. are known from equations (2) and (3). From the geometry of FIG. 2, it is seen that,

sin .theta..sub.2 = h"/r.sub.2 (7)

The radius of curvature r.sub.2 of the exit lens surface 15 is determined by equations (5), (6) and (7).

The above described procedure gives the approximate dimensions of a suitable beam expander. More sophisicated but well-known lens design techniques can be used to obtain the precise optimum shape of the curved surfaces to minimize aberrations and distortions of the wavefront.

The specific embodiment of the invention described with respect to FIG. 2 expands a collimated ultrasonic beam into another collimated beam of increased cross-sectional area. One or both of these beams need not, however, be exactly collimated. In designing a beam expander where either the beam 19 generated by a transducer or the output beam 25, or both, are not collimated, the design considerations discussed herein are equally applicable except that equations (3) and (7), defining the geometry of ray travel, will be different. The equations (4) and (8) derived therefrom will also take a different form.

It may also be noted that the transducer-lens assembly illustrated herein is insensitive to wavelength. That is, the beam emitted by the transducer will be expanded the same amount regardless of the ultrasonic frequency emitted by the transducer.

The use of materials as described herein with widely varying indexes of refraction provide for a compact beam expanding unit. There is a problem, however, in such a combination since the impedances of the materials used differ a great deal which thus results in an impedance mismatch at the incident lens surface 21 and at the exit lens surface 15. As used in the art, acoustic impedance of a material is defined as the product of its density and the speed of sound therein. A mismatch of impedances of two materials creates large energy reflections at an interface between the two materials, as is well known. In the configuration described with respect to FIG. 2, reflective losses at the two interfaces total about 78 percent of the ultrasonic energy striking the incident lens surface 21. Therefore, a large amount of energy is required to be generated by the transducer 17 in order to obtain an output beam 25 with sufficient energy for most applications. To significantly reduce these losses, each surface of the lens 11 is coated with a thin layer of material having an impedance which is nearly a geometric mean between the impedances of the two materials joined. It has been found that graphite has an impedance which is substantially the geometric mean between the impedances of the aluminum lens and the fluids that contact the lens on either side thereof. High density graphite is preferred, having a density in the neighborhood of 1.8 grams per cubic centimeter. The lower density graphite is also satisfactory but is not preferred because of its higher absorption losses. As shown in FIG. 3, the incident lens surface 21 may be coated with a thin layer of high density graphite 31 to substantially reduce reflection losses at the Freon-aluminum interface without significantly affecting the refractive characteristics thereof. Similarly, the exit lens surface 15 may be coated with a thin layer of high density graphite 33 to substantially reduce the reflections of ultrasound at the aluminum-water interface. The reflective losses of such a lens are in the order of 1 percent.

The graphite layers 31 and 33 should have a thickness which is substantially an odd number of one-fourth wavelengths within graphite in order to prevent rather substantial losses therein. Also, since the absolute thickness of the graphite layers should be as small as possible to reduce absorption, it has been found preferable that the layers have a one-fourth wavelength thickness. It should be noted that the thickness of these layers may vary depending on position along a surface of the lens since the thickness must be measured along a sound ray passing therethrough which does not necessarily make the same angle with the surfaces of the layer at all positions. Equations for determining graphite thickness as a function of position along the lens surface are given in the book Waves in Layered Media, L. M. Brekhovskikh, Academic Press (1960). Equation 5.26 at page 50 is especially useful here.

A one-fourth wavelength thickness has a further advantage in situations where the transducer may be operated at either its fundamental or an odd harmonic since a layer having a one-fourth wavelength thickness at the transducer fundamental frequency will have a thickness equal to an odd number of one-fourth wavelengths at any of the transducer's odd harmonics. The typical fundamental frequency for object flaw examination is one megacycle, with the possibility of the same transducer emitting odd harmonics thereof upon adjusting the driving oscillator to 3, 5, or 7, etc. megacycles.

While there has been described a preferred embodiment of the invention, it is understood that further modifications are possible without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

What is claimed is:

1. An ultrasonic lens, comprising,

a solid metal member including aluminum having surfaces shaped to provide desired refraction of ultrasonic wave energy, and

a thin coating of graphite deposited upon said surfaces.

2. An ultrasonic lens according to claim 1 wherein said thin coating of graphite has a thickness substantially equal to an odd multiple of quarter wavelengths of ultrasound for which said lens is designed to be used.

3. Apparatus for generating a controlled beam of ultrasonic energy, comprising,

a solid ultrasonic lens having a convex incident surface and a concave exit surface on opposite sides thereof, said concave exit lens surface being shaped to form a substantially collimated output beam when said exit surface contacts water,

means for generating an ultrasonic energy beam including a flat transducer which generates a collimated beam,

a housing joining said generating means and said lens in a fixed spatial relationship in a manner to direct said ultrasonic energy beam to strike said lens incident surface, and

a coupling liquid sealed in said housing, whereby said liquid transmits said ultrasonic energy from said transducer to said incident ultrasonic lens surface.

4. Apparatus for generating a controlled beam of ultrasonic energy, comprising,

a solid ultrasonic lens having a convex incident surface and a concave exit surface on opposite sides thereof, said solid ultrasonic lens being constructed of a material which is characterized by having a speed of sound therein at least twice that of water,

means for generating an ultrasonic energy beam,

a housing joining said generating means and said lens in a fixed spatial relationship in a manner to direct said ultrasonic energy beam to strike said lens incident surface, and

a coupling liquid sealed in said housing, said coupling liquid being characterized by having a speed of sound therein which is significantly less than that of water, whereby said liquid transmits said ultrasonic energy from said transducer to said incident ultrasonic lens surface.

5. Apparatus for generating a controlled beam of ultrasonic energy, comprising,

a solid ultrasonic lens constructed of a material including aluminum and having a convex incident surface and a concave exit surface on opposite sides thereof,

means for generating an ultrasonic energy beam,

a housing joining said generating means and said lens in a fixed spatial relationship in a manner to direct said ultrasonic energy beam to strike said lens incident surface, and

a coupling liquid sealed in said housing, whereby said liquid transmits said ultrasonic energy from said transducer to said incident ultrasonic lens surface.

6. Apparatus according to claim 5 wherein said coupling liquid includes a halogenated hydrocarbon.

7. Apparatus according to claim 5 wherein said lens incident and exit surfaces are coated with thin layers of graphite.

8. Apparatus according to claim 7 wherein the thickness of said graphite layers is substantially equal to one-quarter wavelength of ultrasound in said graphite for a fundamental frequency of said ultrasonic energy generating means.